If the saddle is at the proper height for you, then I would just ride the bike until a problem develops, if as and when. If you need to adjust the height and there appears to be a problem, then decide. Can't help you with the metallurgical properties of the tubings mentioned. Is the person who gave you the quote a real framebuilder, or "just" another welder? Good luck, but I'd prepare to be disappointed if the tube gives out.